A valve which has been proposed heretofore comprises a main valve body which can be brought against a valve seat by axial displacement in order to close a main passage for flushing liquid. Downstream, the main valve body merges into a cylindrical skirt which surrounds a stationary valve housing provided with a central passage and forms a chamber between the inner end face of the main valve body and the valve housing. This chamber is connected to a pressure probe which is disposed, directed upwards, on the main valve body and projects through a first auxiliary throttle section. Downstream, the chamber leads, behind a second auxiliary throttle section present in the flushing passage, into a region of the flushing passage situated further downstream. The flow connection leading from the chamber into the flushing passage can be closed by means of an auxiliary valve body. When the auxiliary valve body is closed, the difference in pressure forming between the chamber and the outer face of the main valve body and obtained via the pressure drop at the first auxiliary throttle section, causes a closing of the main valve body, but when the auxiliary valve body is open, on the other hand, the reverse difference in pressure obtained via the pressure drop at the second auxiliary throttle section causes an opening of the main valve body.
In this known valve, the main valve body, pressure probe, valve housing and auxiliary valve body are disposed in the centre of the pipe string, while the main passage for the flushing liquid is deflected outwards and surrounds the valve arrangement as an annular chamber. It is not possible for measuring instruments let down through the flushing passage to pass the valve arrangement. Such measuring instruments are used, for example, in the case of a jammed drilling string, to seek the jamming point and the nearest free threaded connection above it, in order to loosen this by means of an explosive charge, under initial tension with a torque directed in the opening sense.
Now, such a valve with the other associated parts of the device for data transmission and the drill stems of non-magnetic material necessary for the magnetic determination of direction represent such a valuable object that in the event of jamming of the drilling string it is worth striving the save these parts. If the jamming point lies below the part of the string which receives the device for data transmission, however, the last free threaded connection must be accessible for this.
It is an object of the present invention to develop a valve of this character which can be passed by the equipment, which can be introduced into the flushing passage and can be reliably actuated by the control device which is restricted to a smaller space as a result of this requirement.